Reflective optical systems with a concave mirror (catoptric), a combination of lenses (dioptric), or a combination of at least one lens and a mirror (catadioptric), are being increasingly used, especially for telephoto lenses. The main reason for using a combined system of lenses and mirrors instead of just lenses is to reduce weight and the cost of optical quality glass required to make large lenses with low f-numbers. It is possible to fold up an optical system into a very compact space by integrating mirrors and lenses.
U.S. Pat. No. 4,061,420 discloses a catadioptric telephoto optical system. The optical system comprises a Mangin mirror as the primary mirror, an aspheric corrector plate wherein a rear surface thereof acts as a secondary mirror, and a pair of air spaced corrector lenses located between the primary and secondary mirrors. In operation, light passes through the aspheric corrector plate and is reflected from the primary mirror and then the secondary mirror. After reflection from the secondary mirror, the light passes through the pair of corrector lenses. and then through an aperture defined by the primary mirror for imaging onto an image plane.
U.S. Pat. No. 4,342,503 discloses various arrangements for two element catadioptric telescopes. A first element comprises a relatively thin light transmissive shell which is concave towards the front of the telescope. A front surface of the first element includes a spherical shape and a rear opposing surface includes an aspheric shape, or vice versa. A central portion of the first element on either the rear surface or the front surface is mirrored to form a secondary mirror of the telescope. A second element comprises a primary mirror which is concave towards the front of the telescope. Additionally. in certain embodiments the second element comprises a light transmissive shell which is spherical on a front surface and aspheric on a back surface, or vice versa. One of these surfaces is mirrored to form the primary mirror of the telescope. In operation, light passes through the first element and is reflected from the primary mirror of the second element and then from the secondary mirror of the first element. After reflection from the secondary mirror, the light passes through an aperture defined in the second element and is imaged on an image plane.
In designing catoptric, dioptric or catadioptric systems, the following advantages and disadvantages of mirrors over lenses must be considered. With reference to the advantages of mirrors over lenses, mirrors can be made in larger diameters than lenses because lenses are essentially limited to a diameter of around 20 inches due to optical limitations. Mirrors, unlike lenses, have no chromatic aberration and, hence, can be focused over the ultraviolet (UV) to infrared (IR) wavelength region. Additionally, a mirror requires only one-quarter of the curvature of a lens having the same power and thereby allows a high relative aperture without introducing excessive aberration residuals.
With regard to the disadvantages of mirrors over lenses. a small diameter centralized mirror obstruction in the path of the entering beam of radiant energy causes some loss of radiant energy collection. Additionally, a reflective surface of a mirror must conform extremely closely to a desired shape because all of the focusing power of the mirror is confined to one surface. Further, a required aspheric surface shape of a mirror is difficult to generate with good optical quality. Still further, an angular field acceptance angle of a mirror system is restrictive. Consequently, the size of the mirror system must be increased by the addition of one or more lens elements.
It is desirable to produce an optical system which reduces the effects of the above mentioned limitations of mirrors and lenses and permits such optical system to be made without use of an aspheric surface.